1. Field of the Invention
The present invention is concerned with a toner used for developing an electrostatic latent image in, for example, electrophotography.
2. Description of the Related Art
In the electrophotography field, the method described in U.S. Pat. No. 229,761 etc. is widely used. In this method, uniform electrostatic charge is applied to a photoconductive insulator (Photocondrum, etc.) by corona charge, and an electrostatic latent image is formed by, e.g., light using various means, and then fine powder, i.e., toner, is electrostatically adsorbed to the latent image to thereby develop the image and make it visible, and if necessary, the toner picture image is transcribed onto a recording medium such as paper, and is fixed to the recording medium by, for example, pressure, heating, solvent vapor blasting or irradiation of light. As the toner used for developing the electrostatic latent image, there are employed particles obtained by pulverizing binder resin made of a natural or synthetic high molecular substance dispersing colorant such as carbon black. Usually a particle diameter of toner is about 5 to 20 .mu.m. These particles are used for the development of an electrostatic latent image as toner alone, or mixed with carrier such as iron powder and glass beads.
The developing methods are classified into a one-component developing method and a two-component developing method. The toner used in the former method usually contains magnetic powder, which is frictionally charged by friction with the wall and the developing roller surfaces and is held on the developing roller by the magnetic force of a magnet incorporated in the roller. The toner is developed to the latent image portion of a photoconductive insulator by the rotation of the roller, whereby the charged toner alone is adhered to the latent image by electric attraction to carry out the development of the image.
In the latter developing method, developer consisting of toner and carrier is frictionally charged by being mixed and stirred in a developing device, and the toner is conveyed to the latent image portion of the photoconductive insulator while carried on the carrier, whereupon the charged toner alone is selectively adhered to the latent image by an electric attraction to carry out the development of image.
As the method of fixing the toner image, although a hot roller fixing method is conventionally employed, a flash fixing utilizing light energy generated by a Xenon lamp is now under development, due to its characteristics as described below.
(1) The flash fixing method does not lower the resolution of the picture image because of the non-contact fixing method.
(2) No waiting time is necessary after the current source is once switched off, and thus an immediate restart is possible.
(3) Even if a recording medium such as copy paper is jammed in a fuser due to a system malfunction, it will not burn.
(4) Any material and thickness of paper can be used for the recording medium, e.g., adhesive paper, preprinted form, and sheet of paper with different thicknesses, etc.
The process by which a toner is fixed to a recording medium by the flash fixing method is explained as follows.
The toner transferred to a recording medium is adhered to the medium in powdered state and forms a picture image. At this stage the image can be destroyed if rubbed with a finger.
When light is irradiated on the picture image by a Xenon lamp, the toner absorbs the energy of the light. The temperature of the toner is accordingly elevated, whereby the toner is softened and melted, and thus is closely adhered to the recording medium.
After the light has been extinguished, the temperature of the toner begins to fall and the toner is thus solidified, whereby a fixed picture image is formed.
A necessary condition for a toner is that the toner resin is softened at a comparatively low temperature, and at the same time, the picture image formed by the toner is not deformed even when the toner is in a molten state.
When a solid, such toner, is melted, however, the viscosity thereof is lowered and the melted toner is coagulated and deformed due to the surface tension thereof. In the present case, the picture image formed by the toner will be deformed.
As binder resin used for the toner, a low molecular weight polymer called oligomer is generally employed due to their low melting temperature and good thermal stability.
The oligomer, however, has defects such that a fixed picture image formed of such an oligomer is easily deformed due to the low melt viscosity and storage stability thereof, and thus the image quality is lowered. Further, when the light energy adsorbed by the toner is too strong, an explosive fixing is liable, to appear white voids called "image void", whereby the photographic density of the image is lowered.
FIGS. 1-(a), (b) and (c) illustrate how a void is formed. These figures show that, when a strong light 3 ((b) of this figure) is irradiated to a toner 1 ((a) of this figure) arranged in multiple columns on a sheet of recording paper 2, the toner 1 is easily melted due to the low softening temperature thereof, and a void 5 is formed inside the toner for the reasons described in the following. Note, 4 in FIG. 1 is a fixed picture image.
When the temperature of a part of toner 1 is elevated to the decomposition temperature thereof, a gas is produced by this decomposition, whereby the part of the toner is protruded, and thus the void 5 is formed.
The air in the empty spaces among the toner particles is thermally expanded and the toner is protruded, whereby a void 5 is formed.
The void 5 formed according to the above mechanism, is formed by explosive fixing.
Even where the toner 1 absorbs energy strong enough to melt, if the melt viscosity and storage stability of the binder resin 1 are too low, compared with the surface tension thereof, the toner aggregate due to the surface tension thereof before the once melted toner 1 cakes, and thus the void 5 may be formed. The shorter the fixation time is, i.e. the faster the printing speed of a printer or a copying machine is, the greater amount of energy irradiated in a short time is necessary to carry out the fixing, and accordingly the above void forming phenomenon occurs more frequently. This phenomenon becomes particularly conspicuous when a high speed machine with a processing speed of 700 mm/sec or more is employed.
As a means of solving the above problems, the molecular weight of a binder resin is merely enlarged. Although the melt viscosity and storage elastic modulus of the toner 1 become higher, the melting point thereof also becomes higher, so the fixability of toner 1 is worsened.
Namely, in a light fixing, as an instantaneous light energy is given to the upper part of the accumulated toners 1. The heat generated by this energy is transmitted to the lower part of toner 1, and the fixing is conducted by the melting of the lower part of the toner (refer to Japanese Patent publication No. 55-140860). Namely, a temperature difference occurs between the upper part and lower part of the toner 1, and the lower part of the toner 1 has a comparatively low temperature. Accordingly, when the melting point of toner 1 is elevated, the lower part of toner 1 is not substantially melted. Thus the fixability is extremely poor. When the thickness of the toner 1 accumulated by developing is greater, the above phenomenon becomes more conspicious. When the thickness of toner 1 after the fixing exceeds 20 .mu.m, a good fixability cannot be maintained. It is, however, difficult to always maintain the thickness of toner 1 to be developed at a constant value.
Further, as the toner 1 for the light fixing, a low molecular resin with a lower softening temperature than the polymeric binder resin to be used in the toner 1 for a hot roller fixing is often employed, and thus a blocking phenomenon may occur such that the toner surface is softened when exposed to high temperature environment and the toners are merged.
When the above blocking phenomenon occurs, the fluidity of the toner 1 becomes extremely low, and not only is the toner not smoothly supplied into the developing vessel but also the particle diameter, etc., thereof changes, whereby the electrification property thereof also changes and a good developed image cannot be obtained.
Therefore, it has been necessary to develop image 1 that exhibits a good fixability even if the amount of toner 1 thereof changes, and in which neither the formation of voids 5 nor a blocking phenomenon will occur.
As described above, in the toner resin for electrophotography in which a light fixing system is employed, as the binder therefor, there are commonly employed an epoxy resin represented by bisphenol A diglycidyl ether, etc. When such a resin is to be used as binder resin, it has been necessary to employ an oligomer with a low softening temperature, i.e. a comparatively lower molecular weight, to show good fixability. Such an oligomer is liable to cause an explosive fixation due to a thermal decomposition thereof, and has a defect such that, owing to the high surface tension and melt viscosity thereof, voids are produced due to the aggregation of the toner particles and the image quality is lowered. Further, the blocking phenomenon occurs when the toner is exposed to a high temperature environment.
To solve these problems, it is necessary to heighten the melt viscosity of a binder resin but not to produce any white voids due to a movement of the binder resin. As a means of heightening the melt viscosity of the binder resin, the following methods are considered:
(1) heighten the polymerization degree of the binder resin. PA1 (2) introduce a comparatively long side chain containing 4 or more carbon atoms into the main chain structure. PA1 (3) introduce a cross-like among the main chain structures of the binder resin.
In methods (1) and (3), however, although the melt viscosity of the binder resin may be heightened, the melting point thereof is also elevated, and although void formation may be prevented, the fixability is often degraded. In method (2), although the melt viscosity of the binder resin may be heightened without elevating the melting point thereof, the blocking resistance is often greatly worsened, and the glass transition point of the binder resin is lowered in this case.